Star trackers can be used to measure the direction towards selected stars. This can be used in a space vehicle, for example, to determine the orientation of the space vehicle.
U.S. Pat. No. 5,745,869 describes a star tracker. The star tracker images a part of the sky onto an image sensor. From the pattern of stars in the image a specific star is identified and from the position of that star in the image the direction of the star is determined.
In order to determine orientation relative to the stars the direction of at least two stars with substantially different directions needs to be determined. Usually a plurality of star trackers is used for this, directed at different parts of the sky. To get an accurate orientation the mechanical orientation of these star trackers relative to a reference frame (e.g. the frame of a space vehicle) has to be accurately known. This requires cumbersome calibration.
A problem in the operation of a star tracker is the effect of stray light from the sun. As used herein, the word star excludes the sun (but it may include planets and other celestial objects, natural or man-made). Apart from imaging objects lenses also scatter a small fraction of incident light in unintended directions. As the light intensity from the sun is much greater than that of stars, scattering of sunlight by the lens can significantly reduce the ability to detect faint stars.
U.S. Pat. No. 5,745,869 describes the use of a sun shade (a baffle) to prevent that light from the sun can reach the lens. The baffle has the shape of a frustrated cone, diverging towards the stars and with its central axis aligned with the optical axis of the lens of the star tracker. As long as the direction of the sun (or other high intensity object such as the earth or the moon) is sufficiently off-angle with the optical axis, the baffle prevents light from the sun to impinge on the lens, thereby preventing scattering by the lens.
The minimum angle distance between the optical axis and the sun needed to avoid scattering depends on the size of the baffle. There is a problem when this minimum angle has to be small, as in this case the baffle must be large.
U.S. Pat. No. 6,199,988 describes an optical instrument with a retractable baffle. The baffle wall has a number of segments. In a retracted configuration the segments lie flat against a surface that is perpendicular to the optical axis. In the active configuration the segments are rotated from the surface, so that they make an angle of less than ninety degrees with the optical axis. In one embodiment the mechanism for rotating the segments has the effect that the cross-section of the segments with a plane perpendicular to optical axis takes the shape of a pointed star, with concave indentations between the points of the star, so that segments of the baffle wall are partly interposed between other segments of the baffle wall. Various other baffle shapes with concave sections are described as well.
In each of these embodiments it is ultimately the length of the baffle segments in the direction of the optical axis that determines the angles of baffled light, although the concave sections may introduce a variation of the baffled range of angles as a function of direction. The patent uses retractable baffles to make it possible to realize segments with greater length, without requiring a large structure in the retracted configuration. Hence, this baffle also has the problem that the baffle must be large when the minimum baffled angle has to be small.
US patent application no 2003/0067680 describes a multi-axis imaging system that is somewhat like an insect's eye, with an array of lenses and a plurality of baffles, each between a respective one of the lenses and a light detector.
In the unrelated art of optical communication devices, U.S. Pat. No. 6,791,073 discloses that an optical communications receiver can be provided with a multi-apertured baffle in the shape of a honeycomb, with walls between the apertures in parallel with the line of sight from the receiver to the transmitter. A telescope, with an optical detector in its focus is located behind the baffle. The multi-apertured baffle reduces the range of direction from which light can be focussed on the detector. As this publication relates to optical communication the receiver contains a simple detector and not an image sensor, nor does the publication address the problem of scattered light.